1. Field of the Invention
This invention relates to a method of producing fine-line circuitry on the surface of a printed circuit board having plated through holes. Specifically, full additive process of circuitization is employed to produce a printed circuit board having fine-line circuitry and increased density. This invention also relates to the resulting printed circuit board and its use in surface laminar circuit technology to form composite structures.
2. Description of Related Art
A conventional process for producing printed circuit boards used in composite structures is as follows: A pattern of holes is drilled through a dielectric substrate or panel. The substrate panel surface and the surfaces created by the holes of the drilled substrate are seeded with a palladium/tin colloidal suspension to catalyze the surfaces for electroless copper plating. Next, the dielectric substrate is copper plated, via electroless or electrolytic deposition, onto the panel surfaces and onto the surfaces created by the through holes at a thickness of approximately 1 mil to about 1.5 mils to form a subcomposite. A minimum thickness copper is required to prevent stress cracking in the through holes. This minimum thickness of copper plating of about 1 mil is often thicker than the desired thickness of the circuitry to be formed on the panel surface, and thus, the subcomposite may be partially etched to reduce the copper thickness on the panel surfaces to about 0.5 mil. Next, the plated through holes are then filled with a compound that can be either conductive or nonconductive. Typically, a layer of negative-acting photoresist is applied to the subcomposite and the photoresist is exposed and developed to define a desired pattern of surface circuitization. The revealed portion of the of copper is then etched away, and finally, the photoresist is stripped to reveal the desired pattern of circuitry.
The subtractive etch method of circuitization, described above, yields a subcomposite having limited wiring density. When conductive metal is etched to form spaces between lines of circuitry, the conductive metal which is covered by photoresist during formation of circuitry, is susceptible to erosion by the etchant in areas beneath the photoresist. To avoid this problem, the thickness of the circuit lines divided by the width of the spaces between them, i.e. the aspect ratio, must be sufficiently low to produce a functional product. The width of spaces limits wiring density, For example, a typical circuit board that has circuit lines about 1 mil thick, requires line and space widths of at least about 3 mils. Also, sharp edge definition, such as the formation of squared lines, is difficult to achieve using the subtractive etch method.
Accordingly, the need exists for a printed circuit board and method of making printed circuit boards, having higher aspect ratio and fine-line circuitry definition on subcomposites with filled plated through holes.
The present invention provides a printed circuit board and a method for the production of a printed circuit board having fine-line circuitry and greater aspect ratio on a subcomposite with plated through holes. The invention herein yields a printed circuit board that has increased wiring and component density. As an additional benefit, the invention herein also provides a printed circuit board and a method of producing a circuit board having landless, plated through holes, and provides yet additional opportunity for increased wiring and component density.
In accordance with the method, a printed circuit board is produced by first drilling a pattern of holes in the dielectric substrate. Next, the substrate undergoes full panel plating, preferably electroless plating, of a layer of conductive metal on the substrate, including the surfaces formed by the holes, at a thickness of approximately 1.0 mil to 2.0 mils to form a subcomposite. The plated holes are then filled with conductive or nonconductive compound, and any compound residue which remains on the surface of the circuit board is removed, preferably by mechanical-chemical scrubbing. The conductive metal layer on the surface of the subcomposite is partially etched to a fraction of its thickness. Next, the nubs of filler compound that protrude from the filled plated through holes are gradually removed, by pumice scrub for example. The subcomposite having conductive-metal plate is etched a second time to completely remove the metal. The remaining nubs of fill compound are chemically polished, preferably, such that a layer of plating will properly adhere to the subcomposite. The resulting subcomposite is a dielectric having filled plated through holes.
Next, the subcomposite is seeded via immersion plating, preferably with palladium/tin, to catalytically activate the surface of the dielectric. A photoresist coating, preferably negative-acting photoresist, is applied to the substrate. The photoresist is exposed and developed to create a fine-line pattern for circuitization. A layer of conductive metal is plated, preferably via electroless plating, on the areas of the subcomposite which are not covered by the photoresist. The photoresist and the seed layer are stripped to reveal the pattern of circuitization which connects the plated through holes. Also, a portion of the circuitry runs through any pads that are present and covering the plated through holes.